Graphite powders and so forth, such as natural graphite, artificial graphite produced by the graphitization of coke, and artificial graphite produced by the graphitization of an organic polymer or pitch, have been used as negative electrode active materials for conventional lithium secondary cells. Organic polymers are added as a binder to these graphite powders, organic solvents or water are added to create a paste, the surface of a copper foil collector is coated with this graphite paste, and the paste is dried to remove the solvent and produce a negative electrode for use in a lithium secondary cell. For example, as disclosed in Japanese Patent Publication No. S62-23433, the problem of internal shorting caused by dendritic precipitation of lithium is solved and the cycling characteristics are improved by using graphite for the negative electrode active material. However, although a lithium secondary cell in which graphite is used for the negative electrode does have better cycling characteristics than a lithium secondary cell in which metallic lithium or a lithium alloy is used for the negative electrode, the following two problems remain unsolved.
The first problem is that the electrolyte decomposes at the graphite surface during initial charging (the first reaction in which lithium is occluded in the graphite). A lithium secondary cell is charged and discharged through the occlusion and release of lithium between the positive and negative electrodes. For instance, in initial charging, if electricity corresponding to 20 units out of 100 units of lithium occluded in the positive electrode is consumed by electrolyte decomposition, this means that only 80 units of lithium end up being occluded in the negative electrode. If there is no electrolyte decomposition, the maximum 100 units of lithium can be utilized in charging and discharging, but in the above example, only a maximum of 80 units of lithium can be utilized, so the electrolyte decomposition reaction in initial charging contributes to lower cell capacity.
The second problem is that with natural graphite particles grown from graphite crystals, which is the to allow greater occlusion and release of lithium, or with artificial graphite particles produced by the graphitization of coke, the interlayer graphite bonds are broken by pulverization, resulting in graphite particles with a higher aspect ratio, which are referred to as flakes. These graphite flakes end up being oriented in the planar direction of a collector when kneaded with a binder and applied over the collector to produce an electrode. As a result, repeated occlusion and release of lithium into and out of the graphite particles causes the graphite layers to expand and contract, creating strain, which decreases adhesion between the oriented graphite particles and the collector, so cycling characteristics and quick charging and discharging characteristics suffer.
In regard to the first problem, suppressing electrolyte decomposition by covering the surface of the graphite with an amorphous carbon layer has been disclosed in Japanese Patent No. 2,643,035. For the second problem, the use of clustered graphite particles so that flat graphite particles will remain unoriented has been disclosed in Japanese Laid-Open Patent Application No. H10-158005, while the use of flake-like natural graphite modified particles having a cabbage-like appearance and a circularity of at least 0.86 has been disclosed in Japanese Laid-Open Patent Application No. H11-263612.